(1) Field of the Invention
The present invention relates to flexible coupling means that can accommodate angular and axial offsets, serving in particular to connect together two mechanical members, for example a transmission shaft and a mechanical part such as a rotorcraft turbine engine.
Consequently, the invention lies in the technical field of transmitting power from one mechanical member to another, in particular mechanical members of a rotorcraft.
(2) Description of Related Art
Most presently-constructed rotorcraft are fitted with at least one free turbine engine. Power is then taken from a low pressure stage of the free turbine, which stage is mechanically independent of the compressor assembly and the high pressure stage of the turbine engine. The free turbine of a turbine generally rotates at a speed lying in the range 20,000 revolutions per minute (rpm) to 50,000 rpm, so a speed-reducing gearbox is needed in order to connect it to the main rotor of the rotorcraft, since it has a speed of rotation that lies substantially in the range 200 rpm to 400 rpm: this is a main power transmission gearbox commonly referred to as the main gearbox (MGB).
Under such circumstances, the turbine engine is connected to the main gearbox of the rotorcraft via at least one transmission shaft that rotates about its own axis of rotation at a speed that is often greater than 5000 rpm.
Similarly, a rotorcraft generally includes a tail rotor in order to compensate the yaw reaction moment due to the main rotor being driven mechanically and in order to enable the rotorcraft to be controlled in yaw.
Consequently, at least one power transmission shaft is arranged between the main gearbox and a tail gearbox for reducing speed in order to drive the tail rotor. Such a transmission shaft must also be capable of rotating at a speed that is greater than 5000 rpm.
Given the speeds of rotation, the transmission shaft needs to be securely fastened by coupling means to the mechanical members it serves to connect together.
Furthermore, the coupling means must enable the transmission shaft to transfer the power developed by one mechanical member to another mechanical member under conditions that are extreme, specifically when the members that are connected together are not correctly aligned relative to each other.
In order to combat the vibration that is generated by the rotating assemblies, rotorcraft are provided with antivibration devices.
Consequently, the main gearbox and possibly even the engine of the rotorcraft are fitted with antivibration devices. Thus, the main gearbox and the engines may possibly be free to move relative to the structure of the rotorcraft, in order to limit the vibration that they generate.
The transmission shafts arranged on those mechanical members must therefore be capable of operating in spite of those movements, in particular vertical movements that give rise to losses of alignment between the members in question relative to each other.
Document FR 1 020 765 discloses coupling means capable of accommodating that type of misalignment.
The coupling means comprise a plurality of annular members of U-shaped axial section. Each annular member includes two vertical plates that face each other and that are provided with a circular attachment.
A first annular member that is provided with a vertical plate provided with a first circular attachment is then connected to a second annular member that is provided with a vertical plate provided with a second circular attachment, by fastening the first circular attachment to the second circular attachment using bolts and lock nuts.
The coupling means are then arranged between two rotary mechanical members, a turbine engine and a transmission shaft, for example.
When one of the mechanical members moves, the coupling means deform so that the mechanical members do not jam. For example, a turbine engine thus continues for example to drive the transmission shaft and thus the main gearbox of a rotorcraft, even in the event of the mechanical members being misaligned. Naturally, the potential misalignment may possibly be restricted to within predetermined limits.
Document U.S. Pat. No. 500,722 presents coupling means provided with two end plates in the form of disks that are connected together by a single curved annular hub.
The coupling means may be made as a single piece. It is possible to obtain those coupling means by forming a groove in a block of material.
Although effective and attractive, it should be observed that those coupling means may be difficult to prepare.
In order to accommodate angular misalignment, the diameter of the hub may be minimized. Nevertheless, that means that the coupling means are no longer capable of transmitting a high level of torque.
One solution then consists in reinforcing the small diameter hub by maximizing its thickness.
Nevertheless such reinforcement has a non-negligible impact on weight.
The technological background also includes the following documents: DE 4 445 413 A1; GB 1 091 121 A; GB 389 907 A; FR 1 536 141 A; U.S. Pat. No. 3,095,714 A; EP 0 459 352 A1; WO 90/10801A1; U.S. Pat. No. 3,988,906 A; and EP 1 936 233 A2.